My pool was rebuilt in 2005. The company installed all new Jandy equipment including a Jandy Stealth JHP 2.0 pump, the Jandy Aqua pure chlorine generator and several of their valves to control the pool/spa flow and the heater/solar flow. Last year I replaced the old filter ( the only thing that wasn't replaced when the pool was rebuilt). It is a Pentair FNS Plus 60. Lately I have been having a problem with the pressure in the filter. It has been going to 40 psi. I have taken the girds out and cleaned them 4 times at this point. I have "descaled" them per the instruction in the manual with a 6 to 1 water/acid mixture. I put the DE in by making a slurry and pouring it into the skimmer ( 6 lbs. ) The filter starts out at about 22 psi with clean DE. After about 3 - 4 hours of operation the gauge goes to 40 psi. I have run the filter with the grids out and it reads 20 psi. If I back wash the filter the pressure goes down to about 22 - 23 again but then goes back up to 40 after a few hours. I have checked the filter gauge accuracy by putting a pressure gauge on the hose bib that was installed between the pump and the filter and get exactly the same reading. I put a flow meter in the line after the filter and am getting a reading of approx 75 GPM with clean grids. The Pentair folks seemed to think that the flow rate of the pump was too much for the filter, which has a max flow of 120 GPM. I have installed a 1 hp motor on the pump and still get the same flow reading. Now I'm being told by Pentair that there may be some "oils or pesticides." I'm not sure how any of these would have gotten into the pool or if they'd cause the problem. Any ideas would be extremely helpful. Thank you.